The present invention relates to the field of computers and, more particularly, to the field of control units and decoders utilized for controlling computers.
In computer systems, a control unit generates a large number of control signals to control the system or functional blocks within the system. The control unit typically includes a decoder unit for providing the control signals.
The control unit sometimes includes hardwired logic and sometimes includes a microprogrammed control store. Hardwired logic units can be cheaper in cost while microprogrammed control stores have greater flexibility.
Microprogrammed control stores operate by having control signals, sometimes called control patterns, read out directly from the control store. Since the contents of the control patterns can be easily changed, a control store provides flexibility whenever the control functions to be performed are to be changed. However, the cost of a control store can be high due to the amount of control memory required to store the control patterns.
Hardwired control logic can provide control patterns using the fewest number of circuits and therefore can have lower cost. However, hardwired logic has the problem of inflexibility. When it is desirable to change the design of hardwired control logic, the design changes are often expensive and time consuming, particularly when the logic is embedded in semiconductor chips. The development stage of a computer using hardwired control logic is difficult because many changes in the control logic occur at this time. Also, after the development stage when the final design is achieved and the product is ready for manufacture, frequently still additional changes are made. These changes either correct previously undetected problems or provide new functions not previously planned. Such changes in the past have proved expensive, difficult to make and time consuming for hardwired decoder units.
In order to overcome the inflexibility of hardwired logic, microprogram control stores are usually preferred because of their flexibility even though they are more expensive than hardwired decoder units.
In accordance with the above background, there is a need for improved control units and decoder units which are low cost and highly flexible.